1. Field of the Invention
The present invention relates to a method and an apparatus for compensating for the irreversible capacity of a negative electrode active material in which in a process of manufacturing a negative electrode for a non-aqueous electrolyte secondary battery using the negative electrode active material with high capacity density, a precursor of the negative electrode is allowed to absorb lithium ions.
2. Background Art
With the widespread use of portable and cordless electronic equipment, the expectation has been increasing for compact, light-weight and high energy density non-aqueous electrolyte secondary batteries. At present, carbon materials such as graphite are practically used as a negative electrode active material for a non-aqueous electrolyte secondary battery. However, the theoretical capacity density of such a material is 372 mAh/g. In order to further increase the energy density of the non-aqueous electrolyte secondary battery, it has been considered to use silicon (Si), tin (Sn), germanium (Ge) and oxides or alloys thereof, which have a higher theoretical capacity density than that of carbon materials. In particular, it has been widely considered to use silicon-containing particles such as Si particles or silicon oxide particles because they are inexpensive.
The above-mentioned negative electrode active materials are incorporated into a battery in a state in which they do not contain a lithium ion unless the materials are particularly subjected to any treatment. A lithium ion contributing to the battery capacity is derived from only the positive electrode active material. In batteries using negative electrode active materials that have not been subjected to any treatment in advance, the irreversible capacity at the time of initial charge is large. Consequently, lithium ions that can be used after the initial discharge are decreased, and thus the battery capacity is reduced. Accordingly, the high capacity density of the negative electrode active material cannot be used satisfactorily.
In order to compensate for this irreversible capacity, attaching a lithium metal foil to the surface of a negative electrode in advance, or forming a lithium metal layer on the surface of a negative electrode by a film formation method in dry processes such as a vacuum deposition method and ion plating have been proposed. Such techniques are disclosed in, for example, International Publication WO 96/27910 pamphlet and Japanese Patent Unexamined Publication No. 2005-038720.
However, since the amount of lithium metal corresponding to the irreversible capacity is very small, when a lithium foil is attached to the surface of a negative electrode, it is necessary to produce an extremely thin foil and attach it. It is difficult to produce such a metal foil and difficult to handle such a metal foil. Therefore, a process of manufacturing a negative electrode becomes complicated. On the other hand, when relatively thick lithium foils are attached to a negative electrode sparsely, the amount of lithium absorbed by the negative electrode active material largely varies in the plane of the electrode. In general, a negative electrode active material having a large capacity density swells according to charging. Accordingly, when a lithium foil is attached in this way, concavities and convexities are generated on the negative electrode and the charge and discharge reaction becomes ununiform. As a result, for example, the charge-discharge cycle property is reduced. Furthermore, when excess lithium metal foils are attached, lithium metals that cannot be absorbed by the negative electrode active material are left on the surface of the negative electrode. At the charging time, dendrites may be generated on the sites. Therefore, there are problems left in terms of the thermal stability and safety.
On the other hand, when a lithium metal layer is formed on the surface of a negative electrode by a film formation method in dry processes, the temperature of the negative electrode rises. This affects the strength of a binder for forming the negative electrode active material layer. When the strength of the binder is lowered, due to the change of stress according to the change of volume of the negative electrode active material at the time of charging and discharging, conductive network between active materials cannot be maintained and the charge-discharge cycle property is lowered. In particular, as mentioned above, the volume of the negative electrode active materials with high capacity density is generally changed according to charge and discharge. Therefore, when such a negative electrode active material is used, the negative electrode active material layer may be easily broken.